1. Field of the Invention
The present invention relates to a loudspeaker system and, more specifically, to a loudspeaker system in which a diaphragm panel is driven by an electromechanical acoustic transducer.
2. Description of the Background Art
Loudspeaker systems in which a diaphragm panel is driven by an electromechanical acoustic transducer have been suggested. One exemplary loudspeaker system employs a scheme in which an electromechanical transducer is directly attached to a diaphragm panel. In another example, a scheme is employed in which a diaphragm panel is acoustically vibrated by an electromechanical acoustic transducer via a space (such a scheme is hereinafter referred to as a sound-driving scheme). Here, the scheme in which the electromechanical transducer is directly attached to the diaphragm panel has several drawbacks. For example, in order to achieve required acoustic characteristics, there is a limitation of the location of the diaphragm panel to which the electromechanical transducer is attached. Therefore, in view of design flexibility of the loudspeaker system, the sound-driving scheme is more advantageous.
FIG. 14 is an illustration showing a basic configuration of a conventional loudspeaker system using the sound-driving scheme. In FIG. 14, 100 denotes a plate-like diaphragm panel. 101 denotes a suspension for supporting the outer rim of the diaphragm panel 100. 102 denotes a frame for fixing the outer rim of the suspension. 103 denotes an acoustic aperture provided on the bottom of the frame 102. 104 denotes an electromechanical acoustic transducer such as to cover the acoustic aperture 103. 105 denotes an enclosed space formed between the diaphragm panel 100 and the electromechanical acoustic transducer 104. In this loudspeaker system, the suspension 101 for supporting the outer rim of the diaphragm panel 100 causes the entire diaphragm panel 100 to perform a piston action for emitting sound. That is, sound emitted from the electromechanical acoustic transducer 104 is led to the enclosed space 105, where air is pressurized to cause the diaphragm panel 100 to vibrate, thereby emitting sound.
It is assumed herein that the diaphragm panel 100 performs a piston action in any frequency band. Under this assumption, an equivalent circuit of the loudspeaker system illustrated in FIG. 14 can be presented as illustrated in FIG. 15. In the equivalent circuit illustrated in FIG. 15, F denotes a driving force of the electromechanical acoustic transducer 104 (driver). Rme denotes a magnetic damping resistance. Cms denotes a compliance of components that support vibrating components of the driver. Mms denotes a mass of the vibrating components in the driver. Rms denotes a mechanical resistance associated with the supporting of the driver. Sd denotes an effective area of a diaphragm of the driver. Furthermore, Cab denotes an acoustic compliance of the enclosed space 105. Rab denotes an acoustic resistance of the enclosed space 105. Cmp denotes a compliance of the suspension 101. Rmp denotes a mechanical resistance of the suspension 101. Mmp denotes a mass of the diaphragm panel 100. Sp denotes an effective vibration area of a diaphragm portion composed of the diaphragm panel 100 and the suspension 101.
As can be known from the equivalent circuit illustrated in FIG. 15, an acoustic transformer is structured based on an area ratio of the effective area Sd of the diaphragm of the electromechanical acoustic transducer 104 with respect to the effective vibration area Sp of the diaphragm portion (Sd/Sp). Therefore, at the time of the operation of the loudspeaker system, an equivalent mass of the diaphragm portion with respect to the electromechanical acoustic transducer 104 is proportional to the square of the area ratio (Sd/Sp). Therefore, if an electromechanical acoustic transducer having a diaphragm area smaller than the diaphragm panel 100 is used, the equivalent mass of the diaphragm panel 100 is small. In this case, even if the diaphragm panel 100 having a large mass is used, the efficiency of the loudspeaker system itself is not degraded.
In the loudspeaker system illustrated in FIG. 14, if a height Tg of the enclosed space 105 is lowered, a reproduction limit frequency in the treble range can be increased. Here, the reproduction-limit frequency in the treble range is defined by the mass Mmp of the diaphragm panel 100 and the acoustic compliance Cab of the enclosed space 105. Also, the acoustic compliance Cab is defined by the capacity and height Tg of the enclosed space 105. Therefore, in order to increase the reproduction-limit frequency in the treble range, the height Tg is lowered, thereby decreasing the acoustic compliance Cab.
FIG. 16 is a graph showing sound pressure frequency characteristics predicted by the equivalent circuit illustrated in FIG. 15. In FIG. 16, the illustrated characteristics can be predicted when the height Tg of the enclosed space 105 is 0.2 (mm), 0.4 (mm), or 0.8 (mm). Conditions for the above prediction are as follows. That is, an electrodynamic loudspeaker whose effective diaphragm area is approximately φ16 (mm) in diameter is used as the electromechanical acoustic transducer 104. Also, a plate of 72 (mm) in height×51 (mm) in width×1 (mm) in thickness made of polycarbonate is used as the diaphragm panel 100. The suspension 101 for use is made of SBR (styrene-butadiene rubber) of 5 (mm) in width×50 (μm) in thickness. As evident from FIG. 16, since the reproduction limit frequency in the treble range is defined by the height Tg of the enclosed space 105, the height Tg has to be lowered in order to increase the reproduction limit frequency in the treble range.
In the above-mentioned conventional loudspeaker system using the sound-driving scheme, a suspension for supporting the outer rim of the diaphragm panel is required. This requirement makes the configuration of the loudspeaker system complicated. Furthermore, the complicated configuration makes it difficult to reduce the size of the loudspeaker system. Therefore, it is difficult to use the conventional loudspeaker system in devices such as portable terminals, which require downsizing and space-savings.
Furthermore, in the conventional loudspeaker system using the sound-driving scheme, it is difficult to improve acoustic characteristics in the bass and treble ranges simultaneously. That is, in the conventional scheme of driving the diaphragm panel by a piston action, the diaphragm panel is required to be high in stiffness and light in weight. However, there is a limitation in order to simultaneously satisfy both of high stiffness and light weight for achieving improvements in the acoustic characteristics. Details are described below.
Descriptions are made below to the fact that lowering the stiffness of the diaphragm panel reduces the sound pressure level. FIGS. 17 and 18 are illustrations showing the results obtained by measuring the characteristics of the loudspeaker system under the same conditions as those of FIG. 16. FIG. 17 is an illustration showing a vibration mode of the diaphragm panel of the conventional loudspeaker system at a frequency of 500 (Hz). FIG. 18 is a graph showing sound pressure frequency characteristics of the conventional loudspeaker system. In FIG. 17, the height Tg of the enclosed space 105 is 0.2 (mm).
FIG. 17 illustrates a vibration mode of the suspension 101 on which the outer rim of the diaphragm panel 100 is mounted. Here, white portions represent a large vibration. As evident from FIG. 17, most of the suspension 101 is greatly vibrated. On the other hand, the diaphragm panel 100 has the outer rim portion being greatly vibrated, and a center portion being slightly vibrated. Therefore, in a bass range at a frequency of 500 (Hz), a separated resonance occurs, that is, the outer rim of the diaphragm panel 100 is greatly vibrated. In other words, in FIG. 17, the diaphragm panel 100 does not perform a piston action, that is, the diaphragm panel is vibrated not as a whole. This is because the stiffness of the diaphragm panel 100 is low. This also means that the equivalent circuit illustrated in FIG. 15 is not applicable. As illustrated in FIG. 18, in practice, the separated resonance in the bass range occurring at the diaphragm panel 100 causes an increase of an acoustic impedance, that is, an acoustic load applied to the diaphragm. As a result, the velocity of the diaphragm is decreased, and the sound pressure level is also decreased. In FIG. 18, a solid line denotes actual measured values of the sound pressure frequency characteristics, while a dotted line denotes predicted values obtained by the equivalent circuit illustrated in FIG. 15. In FIG. 18, the sound pressure level of the measured values is lower than that of the values obtained by the equivalent circuit by approximately 10 (dB).
As described above, when the stiffness of the diaphragm panel is low, the sound pressure level in the bass range is also reduced. In order to solve this problem, the diaphragm panel requires a stiffness to some extent. One way to increase the stiffness of the diaphragm panel is, for example, to configure the diaphragm panel 100 so as to have a sandwich structure, that is, a structure with a core material sandwiched between surface materials attached thereto. Such a sandwich structure of the diaphragm panel 100, however, has several drawbacks. Particularly, the use of the surface materials increases the mass of the diaphragm panel 100, thereby disadvantageously lowering the sound pressure level in the treble range. Furthermore, the sandwich structure of the diaphragm panel 100 is rather a complicated structure, and also increases the thickness of the diaphragm panel 100.
As such, in the conventional sound-driving scheme of causing the entire diaphragm panel 100 to perform a piston action, the stiffness of the diaphragm panel 100 has to be increased in order to improve the sound pressure level in the bass range. In order to improve the treble sound pressure level, on the other hand, the weight of the diaphragm panel 100 has to be reduced. In practice, however, in view of the structure and material of the diaphragm panel, there is a limitation to simultaneous achievement of high stiffness and light weight. Therefore, in the conventional sound-driving scheme, it is difficult to simultaneously achieve improvement in the acoustic characteristics in both the bass and treble ranges.